CD40 is a receptor of the tumor necrosis factor (“TNF”)-receptor superfamily (Banchereau et al., 1994), which is expressed at the surface of B-cells, antigen presenting cells (APC), and several non-hematopoietic cells such as endothelial cells (Hollenbaugh et al., 1995), epithelial cells (Galy & Spits, 1992), fibroblasts (Fries et al., 1995) and keratinocytes (Gaspari et al., 1996). The ligand for CD40 (CD40L) occurs mainly on activated T-cells. Up to now the role of CD40 was mainly studied in the context of the T-cell APC/B-cell interaction (for a review, see Noelle, 1996). Amongst others, the CD40-CD40L interaction seems to be important for the T-cell mediated immunity and for primary and secondary humoral immune response. These findings were confirmed by experiments in mouse models showing that treatment with anti-CD40L antibodies resulted in blocking of the development of mouse equivalents of human autoimmune diseases such as arthritis (Durie et al. 1993), oophoritis (Griggs et al., 1996) and multiple sclerosis (Gerritse et al., 1996).
Activation and transduction through the CD40 pathway within this biological system is mainly responsible for B cell activation and the humoral immune response accordingly.
Apart from NF-κB, factors that can be activated by stimulation of CD40 are NF-AT (Francis et al., 1995) c-Jun, ATF-2 and IRF-1 (Karmann et al., 1996). All these factors play an important role in inflammation.
The CD40L induced signal transduction is, like TNF, mediated by the binding of TNF-Receptor Associated Factors (TRAF's) to the cytoplasmic domain of the receptor. Chaudhuri et al. (1997) demonstrated that, at least in human B cell lines, CD40 and TRAF2 are constitutively associated with each other, and that this association is inhibited by CD40 mediated signals. Apart from the binding with TRAF2, the cytoplasmic domain of CD40, which consists of 62 amino acids at positions 196–257 (mature human CD40-numbering according to Kashiwada et al., 1998), is known to associate with TRAF3, TRAF5, TRAF6 and Janus kinase 3. TRAF6 binds to the amino-terminal cytoplasmic tail of CD40 at positions 210–225, although it can not be excluded that full association of TRAF6 with CD40 may also require the carboxy-terminal part at positions 226–249 (Ishida et al., 1996). TRAF2, TRAF3 and TRAF5 bind to the carboxy-terminal CD40 cytoplasmic domain at positions 226–249 (Ishida et al., 1996).
Stimulation of CD40 results in activation of protein kinases, the mitogen-activated protein kinase and Janus kinase 3/signal transducer and activator of Transcription 3. Moreover, stimulation of CD40 mediates critical biological effects in B cell growth, survival and differentiation.
It is known that TRAF2 and TRAF5 play a role in NF-κB activation in signaling through CD40, as well as TNF-RI, TNF-RII, CD30 and lymphotoxin b receptor. TRAF6 participates in NF-κB activation signaled by CD40 and IL-1 receptor. In addition to these data, International Patent Applications WO 96/16665 and WO 96/28568 disclose a TRAF like protein that binds to the cytoplasmic domain of CD40.